Unless you've installed a new chain recently with an old freewheel/cassette which can cause the chain to slip the most suspicious culprit would be the freewheel/freehub. Here is a video explaining how to lube a freewheel without taking everything apart. What can usually happen is that there are little pawls inside the part that get stuck sporadically when the lube dries out causing it to slip as you describe. Try this, or check some of the other videos and let us know if it worked. You may need some WD-40 or other solvent to free things up if this doesn't work and then apply some real lube afterwards. https://www.google.com/search?q=how+...hrome&ie=UTF-8